udp.7: add UDP_SEGMENT

udp.7: add UDP_SEGMENT

UDP_SEGMENT was added in commit bec1f6f69736
("udp: generate gso with UDP_SEGMENT")

    $ git describe --contains bec1f6f69736
    linux/v4.18-rc1~114^2~377^2~8

Kernel source has example code in tools/testing/selftests/net/udpgso*

Per https://www.kernel.org/doc/man-pages/patches.html,
"Describe how you obtained the information in your patch":
I am the author of the above commit and follow-ons.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Cc: <pabeni@redhat.com>
Cc: <netdev@vger.kernel.org>
[ alx: srcfix + use interval notation ]
Signed-off-by: Alejandro Colomar <alx@kernel.org>